I totally agree with Stillraining. Forget shims, cut out the failed plywood and replace with new wood. I used a short maple plank from an offcuts bin at Rocklers, about $5. On my 1978 I28 I found no rot in the plywood under the compression post but it had distorted badly. Maybe the plywood got wet a few times, maybe the wood wasn't strong enough to begin with and maybe the fact that the foot of the compression post does not sit wholly on the bearer beam contributed. Two cautionary notes: (a) don't jack up the mast too much or you could break the fibreglass tabs holding the bulkhead to the hull and (b) if you're replacing, not shimming, make sure you have some very very solid temporary mast supports - you don't want them giving way! Do not rely on one upright timber balanced on a bottlejack, that would be asking for trouble. I used 6"x2" timbers from Home Depot.

My first boat was an I28Öa fine boat but it had the same problem. It can be fixed permanently but it takes a little work and a few bucks.

I didn't do the work myself, but this is what was done (wish I had taken pictures):

The mast was removed to take all weight off the compression posts. The posts were removed (unfortunately had to remove some of the head bulkhead). Foam filler was epoxied into the bilge between the two stringers fore and aft of where the compression posts were, and glassed over. A section of mast (from a broken mast off another boat) was measured and aluminum plates welded to each end of the new "mast compression post." Wiring for VHF and mast lights were installed inside. Finally, the new compression post was bolted into the glassed section between the stringers. A mast plate was placed on the top of the deck, through-bolted and sealed. Then the original mast was replaced, rig tuned, and everything worked perfectly. I never had any more problems.

Some of you handier types could probably do the work and save some money. Good luck.

I know this is an old post, but here is my story, and the solution I used. When I bought my I-28 I looked it over pretty close and the major flaw I saw was under the sole where the compression post sat. The floor was OK but under it in the bilge was a mess of rotted out plywood.

I was fairly new to boating and everything I had read advised to get a survey before buying. So I called one of the more respected surveyors in the area to look at my prospestive purchase. He came out and looked the boat over (it was in a cradle on land) and about 20 minutes he came out and only noted a few flaws-hose not double clamped -possible blister-crack in the keel joint, BUT NOTHING about the rotted out mast step!! Before he got off the boat I asked him if he had noticed any problems in the bilge, so he went back inside to look. And lo and behold he SAW the problem! He had never lifted the floor boards to inspect the bilge on his initial inspection. Well my confidence in him and other suveyors certainly went down alot.

About 2 weeks later he sent me an adder to his bill ( which I didn't pay) and an elaborate drawing of framing, epoxy and fiberglass to fix the problem. I think if I had a boatyard do this repair it would have been several thousand dollars.

Well. I didn't have that kind of money, so I remembered the KISS principle. Whats been around for thousands of years. doesn't rot, is easy to work with. and is cheap?? CONCRETE!

I bought a sack of concrete, formed it up with some scrap plywood, and 15 years later it still looked like the day I put it in, and the mast had not moved one iota!!

Damned clever, that! It's heavy but low in the boat so it doesn't affect stability. In addition to being rot proof it forms to fit the bottom and spreads the load, too. Did you put in a tube or anything while it was wet so bilge water can move? The only caution I can think of is to make sure it is absolutely dry when the weather freezes.

Yup- I put 3/4 PVC in the bottom and another one higher for wires etc. The concrete was less than 1 square foot and solved the problem. Plus I think the boat was faster after, but maybe it was just my imagination.

There have been quite a few discussions about this issue over the years and searching back in various fora might help. The original design by Bob Perry had a keel-stepped mast so the fault lies with Islander for changing the design during production to create a more open interior to sell boats. I'm inclined to think that most Islander 28s will have this problem which on my boat occurred not only on the cabin sole but also where the metal post sat on the horizontal countertop. The problem is plywood under compression loading perpendicular instead of parallel to the grain. Although the early Islanders were constructed with extremely high quality Bruynzeel plywood, the production Islander simply asked too much of the materials. I jacked mine up with the mast in place and found the process nerve-wracking (expect creaks and groans) but possible. I had to make up about an inch so I replaced the plywood in both locations with stacked 1/4" aluminum plates, one spanning the two cross beams in the bilge and the other a disk between the steel and wooden post at countertop height. I was rightfully reminded to insert a dielectric material between the dissimilar metals on another forum. If I had it to do over again, I would: 1) take the mast down first and, 2) strongly consider giving up the off-center kludge of posts and do what Bob Perry seems to have envisioned. Give up the space and squeeze by a single post centered on the keel. However, this requires advanced boatwright skills and slim and agile sailing companions and for those of us more interested in sailing rather than fixing boats there is always the option of simply ignoring the problem because I don't view it as a critical safety item for the casual sailor. Then again, I am risk-tolerant and I firmly believe that the next meteorite to strike the planet will not hit me. Others may disagree....

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